The invention concerns a self-supporting foil beam-splitter in particular for use in a FTIR (Fourier transform infrared) spectrometer for the far infrared range with an optical thickness on the order of the interesting wavelength.
A self supporting foil beam-splitter of this type, is for example known from the article "Self-Supporting Thin-Film Beam-Splitter for Far-Infrared Interferometer" by G. Kamffmeyer in Zeitschrift Appl. Phys. 14, 313-317(1977).
Beam-splitters are needed in Fourier transform infrared (FTIR) spectroscopy which can split the beam in a ratio of 1:1 over as wide a spectral region as possible. In addition, the beam-splitter must exhibit interferometer grade precision. In the spectral range of ca. 2-25 .mu.m (mid-infrared) beam-splitters are often utilized for this purpose which consist typically of a plane-parallel substrate plate, upon which a thin beam-splitting layer is evaporated, as well as a compensation plate made from the same material and having the same thickness as the substrate plate. The substrate and compensation plates are usually comprised from potassium bromide (KBr) whereas the beam splitting layer, in the simplest case, comprises evaporated germanium (Ge), and the layer thickness is so chosen, that the beam is split as close to the ratio 1:1 as possible, e.g. half is transmitted and half is reflected.
The regions of applicability of such a beam-splitter are limited by the spectral properties of the substrate material and the compensation plate as well as by the efficiency of the beam-splitting layer. Potassium bromide absorbs infrared radiation at wavelengths &gt;25 .mu.m (far infrared). The efficiency of the beam-splitting layer depends on its thickness and its index of refraction. In general the efficiency of a beam splitter is defined by the product of the reflection and transmission (R.multidot.T) which in the most advantageous case with a transmission ratio of 50:50, can assume a value of at most 1/4. In actual cases, however, reflections from every surface are subject to further losses of approximately 4% due to absorption and scattering on the substrate and compensation plates for every beam passage, so that with seven such passages an actual total efficiency of at most (0.96).sup.7 RT is achieved.
Due to the large absorption of potassium bromide in the far infrared region metal grids or thin plastic films have been normally utilized as beam-splitters for radiation with wavelengths .lambda.&gt;25 .mu.m. The efficiency of the metal grids depends on their geometry, in particular on their corresponding grid constant. The metal grids can, in addition, only be usefully utilized for wavelengths which are larger than twice the grid separation.
The plastic films, as described in the above cited article, can be obtained with thicknesses d.gtoreq.6 .mu.m, have however a relatively low index of reflection of n.apprxeq.1.7. Furthermore, the utilized plastics (polyethylene, polyethylene-terephthalate, polycarbonate, polyvinylchloride, polypropylene and the like) exhibit a high absorption in the infrared region and, in particular, also have strong resonance absorption bands which lead to strong prominent structures, caused by the foil beam-splitter used, in the obtained spectra. Due to the low index of the fraction and the strong absorption, the efficiency of these plastic foil beam-splitters is relatively low and these beam-splitters can only be utilized over a small spectral range.
It is therefore the purpose of this invention to present a self-supporting foil beam-splitter for the far infrared region of the above mentioned kind which exhibits a substantially higher efficiency than known plastic foil beam-splitters and has, if possible, no resonance absorption bands in the interesting wave length region of .lambda.&gt;25 .mu.m.